1. Field of the Invention
The present invention relates to a thin film magnetic head used in a magnetic disc apparatus and in particular, to a magnetic recording/reproduction head using the magneto-resistance effect obtained by ferromagnetic tunnel junction.
2. Description of Related Arts
As the magnetic recording apparatus reduces its size and increases capacity, a magneto-resistance effect type head (hereinafter, referred to as an MR head) having a large reproduction output has been used in practice. Such an MR head is already described in "A Magnetoresistivity Readout Transducer", IEEE Trans. On Magn., MAG7, 1971, page 150 [1].
In order to improve the conventional MR head, there has been developed a GMR head using a giant magneto-resistance effect (hereinafter, referred to as GMR) capable of realizing a further increased reproduction output. This GMR uses the magneto-resistance effect called spin-valve effect in which a resistance change corresponds to a cosine of a magnetization direction of two adjacent magnetization layers. This enables to obtain a large resistance change with a small operation magnetic field. The MR head using the spin-valve effect is described in "Design, Fabrication & Testing of Spin-Valve Read Heads for High Density Recording", IEEE Trans. On Magn., Vol. 30, No. 6, 1994, page 3801 [2]. However, the magneto-resistance change ratio obtained by the spin-valve effect that can be applied to an actual MR head is only several percents. When reducing a track width for increasing the recording density, it is necessary to obtain a magneto-resistance effect having a greater resistance change ratio.
The ferromagnetic tunnel junction has a configuration including two ferromagnetic layers sandwiching a tunnel barrier layer made from a thin insulator body having a thickness of several nanometers. In this configuration, when a constant current flows between the ferromagnetic layers while an external magnetic field is applied into the ferromagnetic layers, a magneto-resistance effect phenomenon can be seen. That is, a resistance value is changed according to a relative angle of magnetization directions of the two magnetic layers. This is called a ferromagnetic tunnel junction magneto-resistance effect (TMR). The resistance value is minimum when the magnetization directions are parallel to each other, and maximum when the magnetization directions are anti-parallel to each other. That is, the parallel and the anti-parallel states can be obtained depending on the intensity of the magnetic field. Accordingly, it is possible to detect a magnetic field according to a change of the resistance value.
Recently, there has been developed a TMR element exhibiting a magneto-resistance change ratio near to 20% by forming an Al oxide surface film on the tunnel barrier layer. This increases the possibility to apply the TMR element to a magnetic head and a magnetic memory. An example of such a large magneto-resistance change ratio is described in "Journal of Applied Physics", Vol. 79, 1996, page 4724 to 4729 [3].
That is, using a deposition mask, a first ferromagnetic layer of CoFe is formed on a glass substrate by way of vacuum deposition. Subsequently, the deposition mask is exchanged, and an Al layer is formed to have a thickness of 1.2 to 2.0 nm. This Al surface is subjected to an oxygen glow discharge so as to form a tunnel barrier layer of Al.sub.2 O.sub.3. After this, a second ferromagnetic layer of Co is formed on this tunnel barrier layer over the first ferromagnetic layer, thus completing a cross electrode type ferromagnetic tunnel element. This method enables to obtain a magneto-resistance change ratio as large as 18%.
Various TMR elements are disclosed in Japanese Patent Publication (Unexamined) No. A-5-63254 [4], Japanese Patent Publication (Unexamined) No. A-6-244477 [5], Japanese Patent Publication (Unexamined) No. A-8- 70148 [6], Japanese Patent Publication (Unexamined) No. A-8-70149 [7], Japanese Patent Publication (Unexamined) No. A-8-316548 [8], and "1997 Journal of Japan Applied Magnetism Society, vol. 21, pp. 493 to 496 [9]. These documents give a description on a method in which the Al layer formed is exposed to the atmosphere for epitaxy of Al.sub.2 O.sub.3.
When applying the TMR element to a device such as a magnetic head and memory, it is necessary to reduce affects from a thermal noise. In this case, it is necessary to have a sufficiently low resistance value in practical element dimensions. However, in the conventional tunnel barrier formation method, it is difficult to realize this. Moreover, in application to a magnetic head of high-density design, the signal output voltage becomes a key point. However, with the conventional technique, it is impossible to obtain a sufficiently high density without deteriorating the element characteristic. Furthermore, with the conventional technique, there has been a problem that characteristic fluctuations among elements in a wafer or between lots are too great to obtain a sufficient yield for practical use.
The aforementioned problems are considered to come from the conventional tunnel barrier layer formation method. In the method using the oxygen glow discharge, active oxygen in the radical state or ion is used for oxidation of the conductive layer and accordingly, it is difficult to control the thickness of the oxide film, i.e., element resistance. Moreover, there is a problem that the tunnel barrier layer is contaminated with activated impurities gas generated simultaneously. On the other hand, the method using natural oxidation in the atmosphere also have various problems. For example, a pin hole may be generated in the tunnel barrier layer by dusts in the atmosphere, and the tunnel barrier layer is contaminated wuth humidity, carbon oxide, or nitrogen oxide, similarly in the oxygen glow discharge method.
Japanese Patent Application No. 9-209292 [10] discloses a TMR element production method having a practically sufficient resistance value and signal output voltage characteristic with an improved yield. This method includes: a step for successively forming a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer; a step for forming a conductive layer of a metal or semiconductor; and a step for introducing oxygen into vacuum for natural oxidation of a surface of this conductive layer so as to form a tunnel barrier layer.
Furthermore, Document [10] discloses a TMR element production method for successively forming a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer while maintaining a vacuum. After the first ferromagnetic layer is formed, an oxygen is introduced to oxidize a surface of the first ferromagnetic layer while maintaining the vacuum, and after formation of a conductive layer of metal or semiconductor, an oxygen is introduced into the vacuum for natural oxidization of a surface of this conductive layer to form the tunnel barrier layer.
With reference to FIG. 11, explanation will be given on the TMR film production method disclosed in Document [10]. Firstly, an undercoat layer 10, a first ferromagnetic layer 11, and a conductive layer 12 are successively formed in a vacuum (FIG. 11A). A pure oxygen is introduced without breaking the vacuum, for natural oxidation of a surface of the conductive layer 12 to form a tunnel barrier layer 13 (FIG. 11B). It should be noted that FIG. 11B shows that even after the oxidation of the conductive layer, there is left an un-oxidized portion on the boundary with the first ferromagnetic layer 11. It is also possible to completely oxidize the conductive layer by setting the oxidation conditions as such. After exhausting the oxygen, the second ferromagnetic layer 14 is formed to complete the basic configuration of the TMR film (FIG. 11C). Next, an antiferromagnetic layer 15 is formed to complete the basic configuration of the TMR element (FIG. 11D).
In the a forementioned method, it is possible to obtain an epitaxy of an oxide layer while maintaining a thermal equilibrium in a clean atmosphere not affected by a gas from impurities, thus enabling to control to form a high-quality tunnel barrier layer. Moreover, by controlling the oxygen pressure and the substrate temperature, it is possible to obtain an element of a low resistance and a high current density required for application to a device such as a magnetic head. Furthermore, it is possible to obtain a uniform element characteristic within a wafer and stable repeatability between lots. When the ferromagnetic layer contains Fe, Co, Ni or an alloy containing them and if the conductive layer is made from an Al material having a surface free energy smaller than that of the ferromagnetic layer, it is possible to obtain a preferable coating with respect to the first ferromagnetic layer serving as an undercoat layer. As a result, in an element completed, it is possible to obtain a preferable characteristic having no electrical short-circuit between the ferromagnetic layers due to a pin hole. Moreover, the free energy required for oxidation of Al per one oxygen atom is greater than Fe, Co, Ni, and accordingly, the Al.sub.2 O.sub.3 serving as the tunnel barrier is thermally stabilized on the junction boundary. When Mg or a metal of lanthanoid is selected for the conductive layer, it is possible, from the same reason, to obtain a preferable coating characteristic with respect to the first ferromagnetic layer serving as the undercoat as well as to obtain a thermally stable tunnel barrier.
Referring to FIG. 12, explanation will be given on the conventional TMR element production method disclosed in Document [10]. After formation of the first ferromagnetic layer 11 (FIG. 12A), oxygen is introduced into the vacuum for forming an oxide layer 21 on the surface of the first ferromagnetic layer 11 (FIG. 12B). When forming the conductive layer 12 in the next step, oxygen is diffused from the first ferromagnetic layer 11 into the conductive layer 12 (FIG. 12C), and an oxide layer 23 is also formed at the side of the conductive layer 12. In this method, the oxide layer 24 of the conductive layer 12 is formed on both boundaries in contact with the ferromagnetic layers, enabling to exhibit an excellent thermal stability. After exhausting the oxygen, the second ferromagnetic layer 14 is formed (FIG. 12E) and the antiferromagnetic layer 15 is formed to complete the basic configuration of the TMR element (FIG. 12F). In order to form a stable oxide layer at the side of the conductive layer 12, the conductive layer 12 should have a greater free energy for oxidization per one oxygen atom than an element constituting the ferromagnetic layer 11. When the ferromagnetic layer contains Fe, Co, Ni, or an alloy containing them, it is effective that the conductive layer 12 uses Al, Mg, or a metal belonging to lanthanoid.
FIG. 10 shows a configuration example of an air bearing surface (ABS) of a magnetic head using the conventional TMR element which is described in "Nikkei Electronics", No. 686, Apr. 7, 1997 [11]. The TMR element including a TMR film having electrodes formed at its ends is contained through an insulation film in a magnetic shield.
In the apparatus having the configuration shown in FIG. 10, the TMR element 119 including upper and lower electrode films 118 is present between the magnetic shields which determine the resolution of a reproduction head. Accordingly, it is possible to reduce the distance between the shields while maintaining a sufficient insulation between the TMR element and the upper and lower magnetic shields. However, it is difficult to prepare a thin insulation film and it is impossible to reduce the distance between the magnetic shields smaller than the thickness of the TMR element. Furthermore, for reducing the track width, the electrodes (right and left) need be patterned on the top and bottom of the TMR film so as to make the distance between the electrodes in the order of submicrons, which is quite difficult in the process using a photo-resist. Furthermore, to cope with a noise increase caused by reduction in size of the TMR element, it is necessary to apply a certain bias magnetic field.